JERUSALEM, March 5 (Reuters) - The Israeli navy seized a ship in the Red Sea on Wednesday that was carrying dozens of advanced Iranian-supplied rockets made in Syria and intended for Palestinian guerrillas in the Gaza Strip, the military said.

The disclosure came as Israeli Prime Minister Benjamin Netanyahu was in the United States to press his case for tougher international action against Iran over its disputed nuclear programme and support for Islamist guerrilla groups.

The Panamanian-flagged cargo vessel Klos C was boarded in international waters without resistance from its 17-strong crew in a “complex, covert operation,” military spokesman Lieutenant-Colonel Peter Lerner told reporters.

Lerner said dozens of M302 rockets were found aboard the Klos C, a weapon which could have struck deep into Israel from Gaza and would have significantly enhanced the firepower of the Palestinian enclave’s Hamas rulers and other armed factions.

“The M302 in its most advanced model can strike over 100 miles, and if they would have reached Gaza, ultimately that would have meant millions of Israelis under threat,” he said.

Hamas dismissed the Israeli announcement as a “silly joke”.

“This is a new Israeli lie aimed to justify and prolong the blockade of Gaza,” said Taher Al-Nono, an advisor of Hamas Prime Minister Ismail Haniyeh.

There was no immediate comment from Iran or Syria.

U.S. State Department spokeswoman Jen Psaki said the United States helped Israel by providing information on the ship.

“Soon after becoming aware of the imminent movement of the suspected vessel, the White House directed the Department of Defense to monitor the vessel,” she said.

Psaki said U.S. options for handling the ship included taking unilateral action if necessary but that after sharing intelligence, Israel chose to take the lead in the operation.

Military footage showed the Israeli navy chief, Admiral Ram Rothberg, inspecting a rocket on the floor of a ship hold, with cement bags labeled “Made in Iran” in English next to it.

Lerner said the rockets were flown from Syria to Iran, from where they were shipped first to Iraq and then towards Sudan. Had they reached the African coast, they would have probably been smuggled overland through Egypt to Gaza, he said.

Nic Jenzen-Jones, an Australia-based military arms specialist and director of Armament Research Services, said most reports indicated the Syrian-produced rockets had a 90 to 100 Km range.

“Several Israeli assessments of these rockets have questioned their reliability,” he said. “(Lebanese Shia group) Hezbollah has made use of these rockets, and Hamas is believed to be attempting to stockpile longer-range rocket systems.”

NETANYAHU IN U.S.

Israel and Islamist Hamas last fought a major conflict in November 2012. Hamas has largely held fire since but Israel says it has been trying to build up its capabilities. That has been made difficult, however, by a new military regime in Cairo which has cracked down on the Egyptian border with Gaza.

Netanyahu’s office said that the prime minister, who was in Los Angeles on Wednesday after holding a White House meeting and addressing a pro-Israel lobby in Washington, had approved the ship seizure after consultations with his security chiefs.

“At the same time that it is talking to world powers, at the same time that Iran is smiling and saying all kinds of honeyed words, that same Iran is sending lethal weaponry to terrorist organisations and it is doing so in a complex web of covert, worldwide operations,” Netanyahu said from Los Angeles.

“We followed international law to the letter. The ship travelled under a Panamanian flag, the company was listed in Marshall Islands, the captain was Turkish and the crew was from various different countries,” he told a conference in Tel Aviv.

In a speech to the AIPAC lobby on Tuesday, Netanyahu had reiterated his unhappiness with the prospect that world powers negotiating a curb on Iran’s nuclear programme would let it retain some technologies with bomb-making potential.

The Islamic republic denied seeking nuclear weapons and accused its arch-foe Israel of being the real regional menace.

Iran had orchestrated the shipment, Lerner said, describing the process as months in the making. The Klos C was being brought to the Israeli port of Eilat where its contents would be more fully inspected and displayed to the public.

Lerner said there was no immediate indication the crew had known the nature of their cargo.

According to tracking data, the Klos C was previously at Bandar Abbas port in Iran in early February 2014 and prior to that in Port Said in January. (Additional reporting by Jonathan Saul in London and Nidal al-Mughrabi in Jerusalem and Bill Trott in Washington; Editing by Crispian Balmer, Ralph Boulton and David Gregorio)